Plastic packaging of various electronic devices including semiconductor chips have been employed for some years. Typically an epoxy resin is molded around the chip, a central portion of a lead frame and bonding wires or other connections between contact pads on the chip to inner lead fingers on the lead frame. It is often desirous to decapsulate the package at least in part to allow for inspection, test and repair of the chip and the wire bonds to the chip and inner lead fingers, after at least part of the epoxy covering these elements is safely and effectively removed.
Commonly, concentrated acids such as sulfuric and nitric acids or other solvents for the resin have been employed. Problems in prior art decapsulators and methods include difficulty in controlling the desired amount of etching, in removing debris formed in the etching process, prevention of damage to the package including interior metallization and providing for adequate safety.
An early patent in the decapsulator art is Wensink U.S. Pat. No. 4,344,809 where a jet etch apparatus includes an etching block and a jet pump provides for etchant flow.
A Decapsulator sold by B&G International, Inc. of Soquel, Calif., namely a Model 250, includes an appropriate acid resistant gasket seated on an etch head and the device to be decapsulated is placed on the gasket. A safety cover closes automatically when the process is started, clamping the device in place. The gasket on the etch head allows the chamber to be sealed and pressurized with nitrogen to approximately two PSI. The etching is started when a metering pump moves etchant from a heat exchanger located in plate with a sinuous passage into a cavity formed by the gasket, the device and the etch head. The acid remains in the chamber for a short period of time, where it reacts with or attacks the encapsulant material. After the etchant has been allowed to work, the pump is activated again and a fresh volume of acid is moved into the cavity. This process continues until the desired amount of device exposure is achieved. When the integrated circuit is exposed at the end of the etch cycle, the pump runs continuously to flush the cavity. After this rinsing period the entire system is purged with nitrogen, blowing all waste materials into the waste bottle. At the end of the process, the safety cover opens automatically and the device is removed for post-etch cleaning.
U.S. Pat. No. 5,252,179 discloses a method and apparatus for selective spray etching of an epoxy encapsulated chip. A diaphragm can be raised or lowered to direct flow of etchant, and debris is removed without moving the chip from the chip carrier. Used etchant is collected and recycled. U.S. Pat. No. 5,127,991 shows a process for etching copper sheets in which a pumped source of etchant is heated. U.S. Pat. No. 5,271,798 shows an etching process for tungsten residue on a semiconductor wafer including a sealed apparatus with an etchant inlet port and suction removal of etching byproduct.
In at least some of the prior art devices various deficiencies have been encountered. For example, a fixed etch head is provided which limits the area of the package which is to be etchant attacked; external auxiliary heaters must be used to heat the etchant; excessive etchant fume or seepage limit seal life; there is inefficient removal of etching debris; a particular slowness of the etching process is present due to non-reactive materials at the etch face; and there is a lack of keeping acid consumption low.
In the related application a first acid-resistant syringe pump operates as a positive displacement pump utilizing a standard syringe as the displacement cylinder and piston connected to a first flow valve. To eliminate any problems resulting from non-pulsing flow, a second acid-resistant syringe pump 40 is positioned upstream of the first valve and in connection to a flow line to introduce an oscillating flow of etchant solution. This second pump will, during the etching portion of the process, keep moving acid into and out of an etched cavity being continually formed on the package exterior which is exposed to the etchant, and which action eventually provides the desired decapsulation of the package.
Small outline transistors, commonly known as SOT'S, typically have a length of about 2 mm a width of about 1 mm and a thickness of about 0.5 mm. The epoxy encapsulation of an SOT typically will have a wall thickness of about 0.25 mm. Special problems have arisen with respect to decapsulating these very small electronic device SOT packages. If the etch process is terminated only on the basis of time of etch, the entire device can be digested with attendant damage to the small electronic device including bonding wires and metallization.
Prior art decapsulation etching has used negative pressure on the waste side of the etch head to transport acid to the device being etched. This process automatically terminates when an air leak is created by the side wall of the etched cavity etching through. The use of negative pressure creates a problem by causing the acid to release vapor in the etched cavity. The vapor prevents even etching and reduces the overall etch rate by reducing the efficacy of the acid. This process will leave acid in the lines between the acid control valves and the etch head because of the termination of all acid transport when an air leak is created. This also prevents rinsing the device being etched at the end of the etch process.